Coastlines do not always stay the same shape. As the sea pounds against the shore it gradually wears down the rock to form features such as caves and arches.
• if there is a crack in a cliff, seawater will get into it and gradually hollow out a cave.
• sometimes waves pound both sides of a headland and wear away an arch in the middle.
• Eventually arches collapse, leaving isolated rocks called stacks.
• sometimes waves enlarge a cave so much that a crack appears in the roof. In rough weather water sprays out of the crack, called a blowholes.
In the ocean the water move away from the equator and cold water flowing in from polar regions to take its place.
• The world's largest current is the west wind drift, which flows between American and Antarctica.